1. Field of the Invention
The present invention relates to switching electrical power supplies, and more specifically, it relates to a switching power supply with overcurrent protection.
2. Description of the Related Art
Hitherto, a switching power supply has been provided with overcurrent protection in order to protect the switching power supply itself and a load when an excessive output current flows due to an overload state caused by the occurrence of a short circuit for some reason at the output side of the power supply.
FIG. 4 shows a conventional switching power supply with such overcurrent protection. In FIG. 4, Q1 denotes a primary switching element, which is typically constructed using MOS-FET[s]. A switching control circuit 10 performs on-off control by applying a control voltage to the gate of the primary switching element Q1. An input power supply E is connected to the drain of the primary switching element Q1. The input power supply E is connected in parallel with a smoothing capacitor C1. The source side of the primary switching element Q1 is connected to a rectifier smoothing circuit including a rectifier circuit 2, an inductor 3, and a capacitor C2. A current detection resistor 11 is inserted between the output of the rectifier smoothing circuit and a load 4. The potential difference across the ends of the current detection resistor 11 is input to a current detection circuit 12, from which a voltage signal in accordance with the input potential difference is output to the switching control circuit 10.
If the voltage output from the current detection circuit 12 has a value corresponding to a state in which an output current flowing into the load 4 exceeds a predetermined upper limit value, the switching control circuit 10 limits the ON period of the primary switching element Q1 so as to be shorter or limits the ON period thereof so as not to be longer than the predetermined upper limit value. Thus, the overcurrent protection is performed.
However, in a switching power supply provided with such a conventional current detection circuit, since the current detection resistor (11) for detecting the output current to the load is disposed in a main current path, electrical power loss is generated at this current detection resistor and the power conversion efficiency of the power supply is reduced. Furthermore, since heat is generated due to electrical power loss occurring at the current detection resistor, a large resistor must be used. Accordingly, there are problems in that the overall power supply becomes large and cost reduction cannot be achieved.
The present invention is directed to a switching power supply which solves the foregoing various problems by providing overcurrent protection without using the current detection resistor.
This invention is made by paying attention to a change of a voltage drop caused by a primary switching element in accordance with a current flowing through the primary switching element. That is, a switching power supply is provided with the primary switching element for controlling an output voltage or an output current by switching a current of an input power supply, a current detection circuit for detecting the current flowing through the primary switching element, based on the potential difference across the ends of the primary switching element, and a switching control circuit for controlling the primary switching element so that the current does not exceed a predetermined value.
The current flowing during the ON period of the primary switching element has a correlation, such as being approximately proportional, to the output current through a load, and by detecting the current flowing through the primary switching element, based on the potential difference across the ends of the primary switching element, the output current through the load can be indirectly detected. Accordingly, since a conventional current detection resistor is not required, electrical power loss due to the current-detecting resistor is not generated and problems caused by the heating do not occur.
When the primary switching element is ON, this invention is provided with a timing control circuit for issuing, to the switching control circuit, a current detection signal based on the potential difference across the ends of the primary switching element. When the primary switching element is OFF, the potential difference across the ends thereof is varied in accordance with the circuit construction thereof. However, during the ON period of the primary switching element, the ON resistance (internal resistance) is substantially constant or, due to non-linearity of the primary switching element, the ON resistance (internal resistance) is varied in accordance with the current flowing. This relationship is one in which, as the conductive current increases, the potential difference across the ends of the primary switching element increases. Therefore, regardless of the circuit construction, the switching control circuit can perform overcurrent protection highly accurately by means of switching control based on the potential difference during the ON period of the primary switching element.
In this invention, the current-detecting circuit is constructed to have voltage dividers each provided between the ends of the primary switching element and a ground potential and an amplifier circuit for differentially amplifying the output voltage of each of the voltage dividers, and a temperature-compensating circuit is provided in the voltage divider. Because of this, errors due to temperature characteristic of conductive current versus ON resistance of the primary switching element can be suppressed. For example, since a MOS-FET shows a characteristic thereof that, under the same current, the ON resistance thereof decreases in accordance with an increase in temperature, when temperature compensation is not performed, the current detection value is decreased though the current value is constant during the ON period. However, when current detection is performed based on the potential difference across the ends of the primary switching element in the above-described manner, by applying temperature compensation, a constant overcurrent protection can be performed over a wide range of environmental temperatures.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.